1. Field of the Invention
The present invention relates to an organic light-emitting diode, and more particularly, to a shadow mask which is not sagging, and a method of fabricating an organic light-emitting diode (OLED) display device using the shadow mask.
2. Discussion of the Related Art
One of various flat panel displays, an organic light-emitting diode (OLED) display, emits light by itself. In comparison to a liquid crystal display (LCD) device, the OLED display has the advantageous properties of wide viewing angle and high contrast ratio. It is unnecessary for the OLED display to provide a backlight unit, so that the OLED display realizes thin profile, light in weight and low power consumption.
Furthermore, the OLED display is driven by a low voltage, and the OLED display has a rapid response speed. Also, the OLED display is fabricated with a solid matter, whereby the OLED display can endure the external impact and can be used in the wide scope of temperature. Especially, the OLED display may be fabricated only by deposition and encapsulation apparatuses, so that a method of fabricating the OLED display is simplified.
When the OLED display is driven in an active matrix type where each pixel includes a thin film transistor of a switching element, the same luminance can be realized even in case of applying a low current, thereby obtaining the low power consumption, fineness, and large size of the device.
The OLED display displays images by exciting a fluorescent material using carriers including electrons and holes.
In the meantime, the OLED display is generally driven in a passive matrix type having no additional thin film transistor. However, the passive matrix type has limitations on the lower consumption and lifespan of device. Thus, there are researches and studies for an active matrix type OLED display which is suitable for a next-generation display requiring high resolution and large size.
The OLED display is divided into a lower light-emitting mode and an upper light-emitting mode based on whether an organic light-emitting layer is positioned on a lower substrate or an upper substrate. For example, when realizing the active matrix type in the upper light-emitting mode, a thin film transistor array is provided on the lower substrate and the light-emitting layer is positioned on the upper substrate, it is referred to as a dual plate type OLED (DOD) display.
Hereinafter, a related art OLED display will be described with reference to the accompanying drawings.
FIG. 1 is a cross section view of illustrating a related art OLED display. Referring to FIG. 1, the related art OLED display includes a first substrate 10, a second substrate 20, a thin film transistor array including a thin film transistor (TFT) in each sub pixel of the first substrate 10, an organic light-emitting diode (E) formed on the second substrate 20, and a seal pattern 30 formed in the circumference of first and second substrates 10 and 20. To supply a current to the organic light-emitting diode (E), there are a transparent electrode 16 and a connector 17 which connects the thin film transistor (TFT) to a second electrode 25 by each sub pixel.
At this time, the organic light-emitting diode (E) includes a first electrode 21 which functions as a common electrode, a second electrode separator 26 which is positioned in the boundaries of every sub pixel above the first electrode 21, organic light-emitting layers 22, 23 and 24, and the second electrode 25. In order to form the organic light-emitting diode (E), the first electrode 21, the second electrode separator 26, the organic light-emitting layers 22, 23 and 24 and the second electrode 25 are deposited in sequence, and then the organic light-emitting layers 22, 23 and 24 and the second electrode 25 are separated by the second electrode separator provided on the boundaries of every sub pixel.
At this time, the organic light-emitting layer includes a first carrier-transmitting layer 22, a light-emitting layer 23, and a second carrier-transmitting layer 24, which are deposited in sequence. The first and second carrier-transmitting layers 22 and 24 inject and transport electrons or holes to the light-emitting layer 23.
The first and second carrier-transmitting layers 22 and 24 are determined based on the position of anode and cathode. For example, supposing that the light-emitting layer 23 is selected from a high molecular substance, the first electrode 21 serves as the anode, and the second electrode 25 serves as the cathode. In this case, the first carrier-transmitting layer 22 which is positioned adjacent to the first electrode 21 includes a hole injection layer and a hole transporting layer deposited in sequence, and the second carrier-transmitting layer 24 which is positioned adjacent to the second electrode 25 includes an electron injection layer and an electron transporting layer deposited in sequence.
Also, the first and second carrier-transmitting layers 22 and 24 and the light-emitting layer 23 may be formed of the high molecular substance or low molecular substance. When using the low molecular substance, they are formed by a vacuum deposition method. Meanwhile, when using the high molecular substance, they are formed by an ink jet method.
Unlike a general spacer for the LCD device, a conductive spacer 17 functions as an electric connector between the two substrates as well as cell-gap maintenance. The conductive spacer 17 has a predetermined height between the two substrates.
The thin film transistor (TFT) corresponds to a driving thin film transistor connected to the organic light-emitting diode (E). The thin film transistor (TFT) includes a gate electrode 11 which is formed on a predetermined portion of the first substrate 10, a semiconductor layer 13 which is formed in shape of an island to cover the gate electrode 11, and source and drain electrodes 14a and 14b which are formed at both sides of the semiconductor layer 13. In addition, a gate insulation layer 12 is formed on an entire surface of the first substrate 10, wherein the gate insulation layer 12 is interposed between the gate electrode 11 and the semiconductor layer 13. Then, a passivation layer is formed on the gate insulation layer 12 including the source and drain electrodes 14a and 14b. At this time, the drain electrode 14b is electrically connected to the transparent electrode 16 formed on the passivation layer 15 through a contact hole formed in the passivation layer 15. The upper side of transparent electrode 16 is brought into contact with the conductive spacer 17.
The conductive spacer 17 electrically connects the drain electrode 14b of thin film transistor (TFT) provided by each sub pixel to the second electrode 25 formed on the second substrate 20. The conductive spacer 17 is formed by coating a column-shaped spacer of organic insulation material with a metal material. The sub pixels of first substrate 10 are electrically connected to the sub pixels of second substrate 20 by a one-to-one correspondence.
The metal material for the conductive spacer 17 is selected from a conductive material, preferably, a metal material having the softness and low resistance value. At this time, the first electrode 21 is formed of a transparent electrode material, and the second electrode 25 is formed of a light-shielding metal layer. Also, the interval between the first and second substrates 10 and 20 may be filled with an inert gas or an insulating liquid.
Although not shown, the first substrate 10 includes a scanning line, a signal line crossing the scanning line at a predetermined interval with each other, a power supplying line and a storage capacitor.
For a dual plate type OLED display, there is a bus line formed in shape of a grid on the first electrode 21 of a transparent electrode material having a high resistivity. The bus line prevents a voltage value from being lowered on the first electrode 21.
In the meantime, the organic light-emitting layer is formed on the second substrate 20. The organic light-emitting layer is formed of an organic light-emitting material which emits a predetermined light, for example R, G and B light, for each sub pixel.
FIG. 2 is a plane view of illustrating a shadow mask to form the related art OLED display.
As shown in FIG. 2, the shadow mask 40 is provided with a plurality of cell-forming parts 45, wherein the plurality of cell-forming parts 45 are regularly arranged in the same direction at fixed intervals. Each of the cell-forming parts 45 formed in the shadow mask 40 is provided with a plurality of transmission parts 51 which correspond to R, G and B organic light-emitting layers. At this time, the line width of each of the transmission parts 51 is identical to the line width of each of the R, G and B organic light-emitting layers.
The plurality of transmission parts 51 provided in each of the cell-forming parts 45 are formed in the same direction at fixed intervals. Except the transmission parts 51, the other portions of shadow mask 40 are defined with a masking part 41.
Each of the cell-forming parts 45 is identical in size to one cell corresponding to one OLED display, wherein one cell is provided with a plurality of pixels.
In the process of fabricating the OLED display using the related art shadow mask 40, the organic light-emitting layer is formed on the substrate through each of the transmission parts 51 of the shadow mask 40. At this time, when the shadow mask 40 is held by the evaporation apparatus, and is maintained by a predetermined space from the evaporation apparatus for a preset period of time, the central portion of shadow mask 40 or the other portion adjacent to the transmission part may be sagging due to the gravity.
Especially, as shown in FIG. 2, when using the shadow mask 40 including the plurality of cell-forming parts, a slit type is more advantageous than a slot type having a transmission part in a corresponding pixel in that the slit is easily formed in desired size without decrease in width of the organic light-emitting layer. In this case, the mask is more severely sagged in a length direction of the transmission part due to the large length of slit, wherein the length of the silt is corresponding to that of the transmission part.
The related art shadow mask has the following disadvantages.
When using the shadow mask 40 including the plurality of cell-forming parts, the slit type is more advantageous than the slot type having the transmission part in the corresponding pixel in that the slit is easily formed in desired size without decrease in width of the organic light-emitting layer. In this case, the mask is more severely sagged due to the large length of slit.
The organic light-emitting layer of OLED display is formed by the evaporation deposition method. In this case, the organic light-emitting layer is formed on the portion exposed by the shadow mask. At this time, when the shadow mask 40 is held by the evaporation apparatus, and is maintained by the predetermined space from the evaporation apparatus for a preset period of time, the central portion of shadow mask or the other portion adjacent to the transmission part may be sagging due to the gravity. As the shadow mask is sagging, it is difficult to form the organic light-emitting layer normally. Accordingly, the organic light-emitting layer may be larger or smaller than the desired size, or may be formed in the other portion being apart from the desired portion. Especially, if the transmission parts of shadow mask are provided with the long slits formed at the same direction, the shadow mask may be more severely sagging.
In order to overcome this problem, a magnetic force may be applied to the shadow mask which is formed of the metal material, to thereby lift up the shadow mask. However, there is a requirement for providing an additional apparatus to apply the magnetic force to the shadow mask, thereby increasing the fabrication costs of the device.